<?php

$str = "aaabbbbddeecccf";

function compressString($str)
{
	$num = 1;
	$compressed = "";

	for($i = 0; $i <= strlen($str); $i++)
	{
		if($str[$i] == $str[$i + 1])
		{
			$num++;
		}
		else {
			if($num > 1)
			{
				$compressed .= $str[$i] . $num;
			}
			else
			{
				$compressed .= $str[$i];
			}

			$num = 1;
		}
	}

	return $compressed;
}